ROME (Reuters) - Matteo Renzi is attempting a comeback after an Italian court ruling that paves the way for a national vote this year, but his eventual success may depend on one of his rivals: four-time Prime Minister Silvio Berlusconi.

The 42-year-old Renzi resigned as prime minister in December after his flagship constitutional reform was roundly rejected in a referendum. He immediately called for a vote in 2017, a year ahead of schedule, and Wednesday's Constitutional Court ruling makes elections more probable.

"A national election by June is now very, very likely," Anna Ascani, a lower house lawmaker close to Renzi, told Reuters.

The court ruling scrapped part of the lower house election law and left in its place a proportional system similar to the one that gave Italy 50 governments in 50 years after World War Two. It was abandoned for a largely first-past-the-post system in 1993.

Since Renzi's Democratic Party (PD) is unlikely to win an election outright in a proportional system, it would need to make a post-ballot alliance to secure a majority.

The anti-establish 5-Star Movement, which also calls for an immediate vote, has ruled out alliances.

"The only possible option for Renzi would be to form a government with Berlusconi as a junior partner," said Roberto D'Alimonte, a leading Italian expert on electoral systems.

"Berlusconi is back," he told Reuters.

The 80-year-old Berlusconi, who had open heart surgery last year, cannot run for office due to a 2013 tax fraud conviction. He is hoping the European Court of Human Rights will rule later this year he can be a candidate, and his Forza Italia (Go Italy!) party has said it is in no rush to vote before then.

Forza Italia and the PD already teamed up after the 2013 national election produced no clear winner. Berlusconi abandoned the alliance in 2015 after he clashed with Renzi over who should become Italy's new president.

"RESTART"

Renzi has also not commented since the ruling, but hours before it he created a new blog that portrays him jacketless and smiling casually and begins with the words: "There are many ways to start. And to restart."

Already in December, just a couple of weeks after stepping down, Renzi was plotting his path back to power from a basement office in his home near Florence.

While the court said its ruling had given Italy an immediately applicable electoral law, the voting systems are slightly different for the two houses of parliament.

President Sergio Mattarella, who is the only person who has the power to dissolve parliament, has said they should be aligned before a vote is held.

That is not slowing down Renzi, who wants to vote before his tenure as leader of the fractious PD is due to be put to a divisive vote by the end of the year.

The PD and the 5-Star Movement are currently polling at about 30 percent each. Forza Italia would win about 12 percent, polls say, and the only other party above 10 percent, the far-right Northern League, could never be a PD ally.

"A proportional voting system is a step backwards for Italy, and it will lead to very fragile governments and a very fragile democracy," said D'Alimonte.